1. Field of the Present Invention
The present invention relates to the compound in which light emission is generated through the triplet excited state (hereinafter, phosphorescent compound). Further, the invention relates to an electroluminescent device having an anode, a cathode, and a layer containing an organic compound in which light is generated when an electric field is passed through the layer (hereinafter, electroluminescent layer) and phosphorescent compounds are contained, and a light-emitting device having the electroluminescent device.
2. Description of the Related Art
Organic compounds (an organic molecule) undergo transitions to the energy state having the highest energy (excited state) when they absorb light. Through the excited state, various reactions (photochemical reactions) or luminescence may occur. Hence, there are many applications of the organic compounds.
A reaction of singlet oxygen with unsaturated organic molecules (oxygen addition) is an example of a photochemical reaction. For example, refer to Haruo INOUE, Katsuhiko TAKAGI, Masako SASAKI, and Syosinn BOKU, “Basic Chemistry Course PHOTOCHEMISTRY I”, Maruzen Corporation, pp. 106–110 (1999). Since the ground state of oxygen molecules is a triplet state, singlet state oxygen (singlet oxygen) is not produced by direct photoexcitation. However, in the presence of another triplet excited molecule, singlet oxygen can be produced, and it causes an oxygen addition reaction. Here, compounds capable of producing the foregoing triplet excited molecules are referred to as a photosensitizer.
As stated above, in order to produce singlet oxygen, a photosensitizer capable of producing triplet excited molecules by photoexcitation is required. However, since the ground state of general organic compounds is a singlet state, the photoexcitation to a triplet excited state is a forbidden transition, that is, the probability of generating triplet excited molecules is very small (generally, singlet excited molecules are generated). Therefore, as the photosensitizer, the compounds that are subjected to intersystem crossing between a singlet excited state and a triplet excited state are required. In other words, such compounds are beneficial in use as a photosensitizer.
Further, the compounds that are subjected to intersystem crossing may emit phosphorescence. Phosphorescence is the light emission that occurs from a transition between electronic states of different multiplicities, that is, the light emission occurs from a transition from the triplet excited state back down to the singlet ground state in typical organic compounds. (Further, fluorescence is the light emission that occurs from a transition from the singlet excited state back to the singlet ground state.) Compounds capable of emitting phosphorescence (phosphorescent compound) can be applied to an electroluminescent device containing organic compounds as light-emitting compounds (a device in which light is emitted when an electric field is passed through the device), for example.
When an organic compound is used as a light emitter, the emission mechanism of an electroluminescent device is a carrier injection type. That is, a voltage is applied to an electroluminescent layer interposed between a pair of electrodes, and electrons injected from a cathode and holes injected from an anode are recombined with each other within the electroluminescent layer to produce excited molecules (hereinafter, molecular exciton), then, the molecular exciton radiates energy while returning to the ground state to emit light.
In such an electroluminescent device, an electroluminescent layer is formed generally to have a thin film thickness less than 1 μm. An electroluminescent device does not require backlight, which is used for the conventional liquid crystal display device, since an electroluminescent layer emits light itself, that is, the electroluminescent device is a self-luminous device. Therefore, it is highly advantageous that an electroluminescent device can be fabricated to have extremely thin film thickness and be lightweight.
In an electroluminescent layer with a thickness of about 100–200 nm, it takes several (approximately ten) nanoseconds for the process from injection to recombination of carriers in the light of the carrier mobility. Hence, the time required for the process from injection of carriers to light emission of the electroluminescent layer is on the order of a microsecond even including the process of recombination of carriers. Thus, an extremely high response speed is also one of the advantages.
Further, since an electroluminescent device is a light-emitting device of a carrier injection type, it can be operated at a direct current voltage, thereby noise is hardly generated. With respect to a driving voltage, an electroluminescent layer is formed into an even ultra thin film with a thickness of approximately 100 nm, and an electrode material is selected so as to reduce a carrier injection barrier to the electroluminescent layer. Further, a hetero structure (two-layers structure) is employed. Accordingly, a sufficient luminance of 100 cd/m2 can be obtained at an applied voltage of 5.5 V. For example, refer C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes”, Applied Physics Letters, vol. 51, No. 12, 913–915 (1987).
An electroluminescent device has attracted attention as a flat panel display device of a next generation in terms of its characteristics such as thin shape and lightweight, high response speed, direct current low voltage operation, or the like. In addition, an electroluminescent device is a self luminous device, has a wide viewing angle, and has high level of visibility so that it is considered that the electroluminescent device can be used effectively as a device for the display screen of a portable device.
Further, emission observed in an electroluminescent device is the emission phenomenon which occurs from the transition of a molecular exciton back down to the ground state. Here, a singlet excited state (S*) and a triplet excited state (T*) are examples of types of the molecular exciton formed by an organic compound, which is the same as in the case of photoexcitation. In addition, their statistic generation ratios in an electroluminescent device are considered to be S*:T*=1:3. For example, refer to Tetsuo TSUTSUI, “Textbook of the 3rd seminar at Division of Organic Molecular Electronics and Bioelectronics, The Japan Society of Applied Physics”, p. 31 (1993).
However, light emission (phosphorescence) from a triplet excited state is not observed in typical organic compounds at room temperature. Generally, only light emission from a singlet excited state (fluorescence) is observed. Since the ground state of an organic compound is generally a singlet ground state (S0), the transition of T* to S0 (phosphorescence process) is an intense forbidden transition, and the transition of S* to S0 (fluorescence process) is an allowed transition. That is, light emission generally occurs from only a singlet excited state.
Therefore, the theoretical limit of internal quantum efficiency (the number of photons released per carriers injected) of an electroluminescent device is considered to be 25% based on the fact that S*:T*=1:3.
Also, light generated is not entirely emitted to the outside, and a part of the light cannot be extracted because of component materials of an electroluminescent device (electroluminescent layer materials, electrode materials) or the substrate materials-specific refractive index. The efficiency of extraction of generated light is referred to as light extraction efficiency. The light extraction efficiency of an electroluminescent device having a glass substrate is said to be approximately 20%.
For these reasons, even if all injected carriers form molecular exciton, the theoretical limit of the ratio of photons extracted finally emitted to the outside to the number of injected-carriers (hereinafter, external quantum efficiency) has been said to be 25%×20%=5%. That is, even if all carriers are recombined, only 5% thereof can be extracted as light emission.
However, in recent years, an electroluminescent device in which energy released from a transition of a triplet excited state (T*) back down to the ground state (hereinafter, triplet excited energy) can be converted into light emission has been reported successively. The high luminous efficiency has attracted attention. For example, refer to D. F. O'Brien, M. A. Baldo, M. E. Thompson and S. R. Forrest, “Improved energy transfer in electroluminescent devices”, Applied Physics Letters, vol. 74, No. 3, 442–444 (1999), referred to herein as “O'Brien et al.” For another example, refer to Tetsuo TSUTSUI, Moon-Jae YANG, Masayuki YAHIRO, Kenji NAKAMURA, Teruichi WATANABE, Taishi TSUJI, Yoshinori FUKUDA, Takeo WAKIMOTO and Satoshi MIYAGUCHI, “High Quantum Efficiency in Organic Light-Emitting Devices with Iridium-Complex as a Triplet Emissive Center”, Japanese Journal of Applied Physics, Vol. 38, pp. L1502–L1504 (1999), referred to herein as “Tsutsui et al. (vol. 38).”
In O'Brien et al., a porphyrin complex containing platinum as a central metal is used. In Tsutsui et al. (vol. 38), an organometallic complex containing iridium as a central metal is used. Both of the complexes are phosphorescent compounds containing 3rd transition series elements as central metals. Some of them have a phosphorescent compound having external quantum efficiency beyond the above mentioned theoretical limit value, 5%.
Further, a layer comprising an organometallic complex (iridium complex) containing iridium as a central metal and a layer comprising DCM2, a known fluorescent compound, are stacked alternately to transfer triplet excited energy generated in the iridium complex into the DCM2. Consequently, light emission of the DCM2 also can be excited, as in M. A. Baldo, M. E. Thompson and S. R. Forrest, “High-efficiency fluorescent organic light-emitting devices using a phosphorescent sensitizer”, Nature (London), vol. 403, 750–753 (2000), referred to herein as “Baldo et al.” Light emission of DCM2 is the light emission (fluorescence) occurring from a singlet excited state. The electroluminescent device having the foregoing device configuration has the advantage that triplet excited energy of an iridium complex generated efficiently can be utilized for singlet excited energy of the DCM2 which is another molecule. As one might say, the iridium complex used as a phosphorescent compound serves as the above-mentioned photosensitizer to increase the ratio of singlet excited state of the DCM2.
As disclosed in O'Brien et al., Tsutsui et al. (vol. 38) and Baldo et al., the organic compounds in which light emission is generated through the triplet excited state, that is, an electroluminescent device containing phosphorescent compounds can achieve higher external quantum efficiency than that of the conventional device. High external quantum efficiency leads to improved luminance. Therefore, an electroluminescent device containing phosphorescent compounds will play a great role in the development of an electroluminescent device in the future as means for improving luminance and luminous efficiency.
Accordingly, phosphorescent compounds are expected to be used as a photosensitizer or a material for an electroluminescent device because the phosphorescent compounds are subject to intersystem crossing and emit light (phosphorescence) from the triplet excited state. However, the number of applicable phosphorescent compounds is few under the existing circumstance.
An iridium complex, in a few phosphorescent compounds, disclosed in Tsutsui et al. (vol. 38) or Baldo et al., is one type of organometallic complexes referred to as an orthometallated complex. The iridium complex has a phosphorescence lifetime of several hundred nanoseconds and high phosphorescence quantum efficiency so that the decrease in efficiency as luminance is improved is less than that of the porphyrin complex. With such a perspective, the orthometallated complex containing such a heavy metal serves as a guide to synthesize phosphorescent compounds.
The structure of the ligand of the iridium complex disclosed in Tsutsui et al. (vol. 38), which is comparatively simple and gives green emission with good color purity, should be altered in order to vary the emission color to another. For example, it is disclosed that some emission colors were achieved by synthesizing various ligands and iridium complexes containing the foregoing various ligands in M. Thompson, S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, S. R. Forrest, M. Baldo, P. E. Burrows, C. Adachi, T. X. Zhou and J. J. Brown, “Phosphorescent Materials and Devices”, The 10th International Workshop on Inorganic and Organic Electroluminescence (EL '00), 35–38.
In addition, many of the foregoing ligands have difficulty in synthesis or the number of processes for the synthesis is large, and it causes an increase of cost. In order to emit phosphorescence in an orthometallated complex, it is necessary to use iridium or platinum as a central metal; however, these metal materials are expensive, and it causes an increase of cost of the ligands eventually. Further, blue emission with good color purity has not been realized yet.
Therefore, it is required to explore various emission colors by synthesizing a novel orthometallated complex (phosphorescent compound) by using ligands that can be synthesized easily.